1. Field of the Invention
This invention relates to a sheet forming apparatus and a method of controlling the same, specifically to a sheet forming apparatus employing a touch roll, which allows a sheet to go through between two rolls in such a manner that the front face and the back face thereof are abutting the rolls, respectively, thereby to form a sheet, and a method of controlling such a sheet forming apparatus.
2. Description of the Related Art
As a sheet forming apparatus relying on an extrusion molding method, there is an apparatus that has two rolls disposed in parallel with each other, leaving a gap therebetween, wherein a sheet (a molten resin from a T die) goes through between the two rolls that are driven to rotate while the front and the back face of the sheet are abutting the rolls, respectively. (See Japanese Patent Application Laid-open Publications Nos. H09-155948 and H10-34748.)
In a related art sheet forming apparatus, a wedge member is inserted between two roll supporting housings (bearing housings) that respectively support the two rolls and one of the roll supporting housings is pressed onto the other housing by use of an oil-pressure cylinder apparatus, wherein a roll gap is controlled through an insertion depth of the wedge member into between the two roll supporting housings.
In this sheet forming apparatus, since the wedge member is inserted between the two roll supporting housings, it is impossible to measure quantitatively the force (load) applied onto a sheet being formed, but only possible to maintain the roll gap constant.
When an extrusion amount of a molten resin from a T die is increased at the time of starting an operation (or changing forming conditions), the roll gap has to be widened. However, if it is delayed for the gap to be widened, there is formed a larger melt bank on the upper roll. An abnormally large melt bank overloads a roll driving motor, or may cause surface defects such as bank marks and the like. In addition, the molten resin of the melt bank adheres on a T die lip and smears the same.
On the other hand, when an extrusion amount of a molten resin from a T die is decreased at the time of starting an operation (or changing forming conditions), the roll gap has to be narrowed. However, if it is delayed for the gap to be narrowed, the rolls do not touch a sheet being formed. In this case, the sheet is not cooled sufficiently by the rolls. Therefore, the sheet is no longer hardened enough, thereby resulting in a soft sheet, which may allow such a sheet to leave of from the rolls at the lower portion thereof and to fall down.
In the past, the roll gap has been manually adjusted by an operator of such a forming apparatus. Namely, he or she adjusts the roll gap through observation of a size of the melt bank while controlling the extruding machine and a line speed. As stated above, if the roll gap is excessively wide, the sheet cannot be cooled enough and a part thereof hangs down to be wound around the roll. On the other hand, if the roll gap is excessively narrow, the melt bank grows large, which causes surface defects such as bank marks on the sheet and overloads the roll driving motor into halt. Therefore, the forming apparatus needs to be continuously adjusted by the operator.
In the past, there has once been employed a method in which a sheet take-off speed is changed simply in proportion to a rotation speed of a screw of the extruding machine, which is known as a so-called simple proportion operation. In this method, an extrusion amount of a molten resin from the extruding machine is adjusted in retard of a change in the rotation speed of the screw. Therefore, when the rotation speed changes quickly, the roll gap changes before the extrusion amount changes in accordance with the speed change, which leads to such troubles of sheet hanging, motor overload, or the like. This makes it almost impossible to change the screw speed quickly.